In optical lithography, a desired pattern may be created on a substrate by providing a resist layer on the substrate and exposing this layer to a patterned beam of radiation. The pattern thereby created in the resist layer may be used in subsequent procedures (e.g. etching, metal deposition, etc.) to create a desired structure in or on the substrate, for instance a microcircuit structure. Patterning the beam of radiation may be done, e.g., with a reticle or a micromirror device, and generally the patterned beam of radiation is first guided through a lens system before reaching the substrate.
Conventionally, the lens system was such that each lens would sequentially receive the patterned beam of radiation, i.e. the beam was first passed through (or reflected by) lens 1, then through (or reflected by) lens 2, etc. More recently, however, it has been proposed to further add to certain systems a set of lenses constructed and arranged such that the cross section of the patterned beam impinges on a plurality of lenses in the set. An example of such a set of lenses is for instance a microlens array (MLA). See, e.g., U.S. Pat. No. 6,133,986.
A disadvantage of using conventional MLAs in lithographic apparatus, however, is that the lenses are not individually controllable. E.g., if several of the lenses in the MLA are out of focus (for instance because the substrate is warped or because of errors in manufacturing the MLA), it is cumbersome if not impossible to correct for this focal deficiency without negatively affecting the focus of the other lenses in the MLA.
Therefore, what is needed is an exposure system comprising a set of lenses, wherein the set of lenses comprises variable lenses. Furthermore, what is needed is a set of lenses comprising variable lenses.